The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Reverse osmosis systems are used to provide fresh water from brackish or sea water. A membrane is used that restricts the flow of dissolved solids therethrough.
Referring now to FIG. 1, a reverse osmosis system 10 is illustrated having a membrane array 12 that generates a permeate stream 14 and a brine stream 16 from a feed stream 18. The feed stream 18 typically includes brackish or sea water. A feed pump 20 coupled to a motor 22 pressurizes the feed stream 18 to the required pressure flow which enters the membrane array 12.
The permeate stream 14 is purified fluid flow at a low pressure. The brine stream 16 is a higher pressure stream that contains dissolved materials blocked by the membrane. The pressure of the brine stream 16 is only slightly lower than the feed stream 18. The membrane array 12 requires an exact flow rate for optimal operation. The flow rate provides a specific pressure for optimization. A brine throttle valve 24 may be used to regulate the flow through the membrane array 12. Changes take place due to water temperature, salinity, as well as membrane characteristics, such as fowling. The membrane array 12 may also be operated at off-design conditions on an emergency basis. The feed pumping system is required to meet variable flow and pressure requirements.
In general, a higher feed pressure increases permeate production and, conversely, a reduced feed pressure reduces permeate production. The membrane array 12 is required to maintain a specific recovery which is the ratio of the permeate flow to feed flow. The feed flow or brine flow likewise requires regulation.
Referring now to FIG. 2, a system similar to that in FIG. 1 is illustrated with the addition of a feed throttle valve 30. Medium and large reverse osmosis plants typically include centrifugal-type pumps 20. The pumps have a relatively low cost and good efficiency, but they may generate a fixed pressure differential at a given flow rate and speed of rotation. To change the pressure/flow characteristic, the rate of pump rotation must be changed. One way prior systems were designed was to size the feed pump 20 to generate the highest possible membrane pressure and then use the throttle valve 30 to reduce the excess pressure to meet the membrane pressure requirement. Such a system has a low capital cost advantage but sacrifices energy efficiency since the feed pump generates more pressure and uses more power than is required for a typical operation.
Referring now to FIG. 3, another system for solving the pressure/flow characteristics is to add a variable frequency drive 36 to operate the motor 12 which, in turn, controls the operation of the feed pump 20. Thus, the feed pump 20 is operated at variable speed to match the membrane pressure requirement. The variable frequency drives 36 are expensive with large capacities and consume about three percent of the power that would otherwise have gone to the pump motor.
Referring now to FIG. 4, a system similar to that illustrated in FIG. 1 is illustrated using the same reference numerals. In this embodiment, a booster pump 40 having a pump portion 42 and a turbine portion 44 is used to recover energy from the brine stream 16. The pump portion 42 and the turbine portion 44 are coupled together with a common shaft 46. High pressure from the brine stream passes through the turbine portion 44 which causes the shaft 46 to rotate and drive the pump portion 42. The pump portion 42 raises the feed pressure in the feed stream 18. This increases the energy efficiency of the system. The booster 40 generates a portion of the feed pressure requirement for the membrane array 12 and, thus, the feed pump 20 and motor 22 may be reduced in size since a reduced amount of pressure is required by them.
Referring now to FIG. 5, a basic low-cost scheme for a large reverse osmosis plant 50 is illustrated using reference numerals similar to those of FIG. 1. In this embodiment, three reverse osmosis stages having three membranes 12a, 12b, and 12c are used together with three throttle valves 30a, 30b, and 30c. The three membranes 12a, 12b and 12c generate respective permeate streams 14a, 14b, and 14c. Three brine throttle valves 24a, 24b, and 24c are coupled to the brine output 16a, 16b, and 16c. The feed stream 18 is coupled to a feed manifold 52 which, in turn, is coupled to each of the feed throttle valves 30a-30c. Each throttle valve is used to provide feed fluid to each of the respective membrane 12a-12c. The brine stream 16a-16c passes through the brine throttle valves 24a-24c and into a brine manifold 54. The permeate streams are coupled to a permeate manifold 56.
In a large reverse osmosis plant 50, the objective is to use a feed pump with the largest available capacity to achieve the highest possible efficiency at the lowest capital cost per unit of capacity. The optimal capacity of a membrane array 12 is usually smaller than the pumps. Therefore, a single-feed pump 20 may be used to multiple supply membrane arrays 12. Such a configuration is called centralized feed pumping. Because each of the membranes has a variable pressure requirement, individual control using the throttle valves 30a-30c and 24a-24c may be used. However, using throttle valves wastes energy. Also, the individual membranes themselves may have their own pressure requirements due to the following level of the membranes which may vary over the membrane array.
Referring now to FIG. 6, a similar configuration to that of FIG. 5 is illustrated with the addition of a variable frequency drive used to drive the motor 22 and thus the pump 20. The three membranes 12a, 12b and 12c generate respective permeate streams 14a, 14b, and 14c. The variable frequency drive 60 is used to develop enough pressure at the pump 20 to satisfy the pressure requirements of the membrane arrays with the highest pressure requirement. The centralized pumping is partially offset by the difficulty of customizing the fixed discharge pressure of the feed pump to the variable pressure requirements of the multiple membrane arrays. Both of the configurations in FIGS. 5 and 6 require individual throttling and, thus, the energy efficiency is limited.
Referring now to FIG. 7, an embodiment from U.S. Application WO2004/065308 is illustrated using the same reference numerals as FIG. 1 for the common components. In this embodiment, a hemi 70 includes a booster 72 having a pump portion 74 coupled to the feed stream 18. The booster 72 also includes a turbine portion 76 coupled to the brine stream 16. A motor 78 is coupled to the common shaft 80 that is used to drive the pump portion 74 and the turbine portion 76. A variable frequency drive 82 may be used to drive the motor 78 in a similar way to that illustrated in FIG. 3. However, in this embodiment, the variable frequency drive 82 controls the motor 78 which turns the common shaft 80 of the pump portion 74 and the turbine portion 76. In this embodiment, a second booster 84 having a second turbine portion 86 and a pump portion 88 may be used together with a motor 90. The turbine portion 86 receives brine fluid through the brine throttle valve 24 which turns the common shaft 92 which generates input pressure through the pump portion 88. The pump portion may be used to generate pressure on fluid received from the reservoir 94. Fluid is returned from the reservoir through the turbine portion 76 and a turbine portion 86. The pump portion 88 acts as a primary feed pump for the system. The pump portion 74 provides a main source of energy recovery for the system. The flow of energy through the feed stream 18 may be changed using the throttle valve 24 to divert some of the brine stream to the turbine portion 86.